Reactor vessels employed in the nuclear power industry, as well as similar vessels used with large industrial facilities in general are fabricated as welded plate structures. Typically, reactor vessels will be formed with longitudinal and circumferential seam welds, as well as nozzle welds and the like at their cylindrical or main body portions and with corresponding welds at their top and bottom heads. Because of the criticality of maintaining the structural integrity of power reactor vessels over their somewhat extended lifespans, regulatory agencies such as the Nuclear Regulatory Commission (NRC) require extensive examination of the welds within predetermined intervals. Typically, non-destructive examination and evaluation of the welded structures are carried out during scheduled shut-downs planned for such activities as refueling and the like.
Because such planned shut-downs involve a power production outage, the efficiency of their execution is most important to industry. However, the weld inspection procedure is complex, requiring control over man-rem exposure, and thus calling for remotely controlled examination systems which themselves must be capable of operating within the environment of gamma radiation. Where boiling water reactors (BWR) are the subject of inspection, advantages have been recognized for an internal approach wherein the water media within the reactor vessel or additionally that within the refueling cavity serve to isolate personnel from radiation originating from the nuclear fuel. Remotely controlled manipulators generally are employed to physically move and position inspection heads or search units carrying ultrasonic inspection transducers and the like to positions adjacent the various vessel weldments. Ultrasonic test (UT) examination then is carried out under the control of remote stations which may be located as far is several hundred feet from the manipulator carried search units. In locating weld flaws, piezoelectric crystal based transducers are excited, preferably at their resonant frequencies by a remotely generated pulse delivered from along long lengths of shielded cable. The same or another such transducer retained crystal then reacts to a received echo of much lower signal level to form an evaluating signal which is returned along a lengthy communications cable for data acquisition at the remote control station. To achieve adequate excitation of the transducers, resort generally is made to relatively higher level, d.c. power supplies, for example switching supplies in the 300 Vdc to 1,000 Vdc range. These power supplies, in and of themselves, are sources of undesirable noise and much of the excitation energy generated by them will be dissipated by the shielded transmission cabling employed for its delivery. Cable terminations must be precisely installed to avoid interfering signals arising due to energy reflections up and down the length of the cables. Signal loss conditions are further experienced with respect to the low signal level responses transmitted for data acquisition for the same reasons in addition to the attenuation of the cable acting upon very low level signals. Important information can be lost as ultrasonic signals are attenuated to below the electronic noise level of the system.
Noise phenomena present within the reactor facility also have been seen to impact upon the quality of the low level signals arising due to targets in the specimen under inspection. Nuclear power reactor installations and other industrial facilities generally are constructed having a plant grounding system including a buried electrical base mat or grid to which various plant components are electrically coupled. The reactor pressure vessel will be coupled along one path to ground as will plant machinery. Similarly, electrical control instrumentation will be grounded through a segregated linkage. Notwithstanding the presence of these grounding systems, components such as the reactor pressure vessel typically exhibit ground potentials varying several volts, for example, from instrumentation grounds. As a consequence, an opportunity for ground potential based noise generation is present. This has called for elaborate structuring and positioning procedures for the transducer-carrying search units to avoid development of any electrical communication between the transducer and its associated electronics with the reactor vessel wall and structuring connected thereto.
From the foregoing, it may be observed that improvements in the techniques for transducer operation, both in noise avoidance and in excitation and echo response approaches will be well received by many industries. In addition, any such improvements in the nuclear industry must be evolved with instrumentation or circuitry which is reliable under the harsh operating environment posed by gamma radiation if the previously discussed limitations on currently available instruments are to be overcome. Any loss of reliability will be manifested by costly time losses occurring with shut-down maintenance activities and attendant lengthening of the interval of outage.